– Pregnant women who’ve traveled to Latin America and the Caribbean are advised to be tested for the Zika virus afterward. But medical researchers have discovered there’s a problem with that advice: Some diagnostic tests will return positive results even when a person hasn’t contracted Zika.

That ambiguity can force pregnant women who fear giving birth to babies with severe brain damage to make life-changing decisions based on incomplete information.

The discovery by Zika researchers that current antibody tests don’t distinguish between Zika and dengue, another mosquito-borne virus, is the latest twist in the scientific world’s confrontation with a virus long thought relatively harmless but now thought able to cause serious birth defects as well as life-threatening complications in adults.

“If I’m pregnant and I’m trying to figure out whether my baby is in danger or not, it makes a huge difference whether it’s Zika or dengue. Because dengue doesn’t cause microcephaly. That’s where it’s important. It’s a big deal,” said Ilhem Messaoudi, an immunologist at the University of California, Riverside, who studies viral infections and maternal and fetal health.

Speaking at the 46th annual Washington Conference on the Americas, an event hosted by the Council of the Americas at the State Department last week, Carissa Etienne, the director of the Pan American Health Organization, described developing accurate antibody tests as one of the next fronts in the fight against the rapidly spreading Zika virus.

Etienne said reliable data was crucial considering that so much was still unknown about Zika. But getting that information can be difficult because other viruses, such as dengue and chikungunya, are prevalent in the same areas, and scientists don’t understand how the infections interact.

“We have challenges,” she said. “We have challenges in that we do not have a diagnostic test that can help us.”

Since the first case was discovered in Brazil last year, Zika has been found in 36 countries and territories in the Americas. The World Bank estimates that the economic impact in Latin America and the Caribbean is likely to reach $3.5 billion this year.

In the United States, there have been 472 cases in 47 states and the District of Columbia, Puerto Rico and the U.S. Virgin Islands. U.S. researchers worry that Zika poses a greater risk than had been thought after confirming that a mosquito species known as the Asian tiger, more common in cooler climates, could spread the virus deeper into the country. Previously, the Aedes aegypti, which thrives in warm climates, was thought to be the primary transmitter of the virus.

Etienne said researchers at her organization were working on developing an agenda to help countries research the problem and come up with a test that could distinguish between dengue and Zika.

There is a test to diagnose Zika if it’s administered within six days of infection. But that is of little value to women who discover they are pregnant weeks after traveling or are uncertain when they became pregnant.

The other test, which examines proteins that fight previous infections, does not distinguish between Zika and dengue. Zika, dengue and West Nile, prevalent in the Caribbean and Latin America, are part of the same flavivirus family, and the antibodies the body uses to fight off the viruses are also similar.

“They’re cousins,” Messaoudi said. “So as cousins you have some similarities.”

There is no complete information on how many women who’ve discovered they have had Zika have elected to terminate their pregnancies. Abortion is illegal in many of the most heavily infected areas.